It is generally known to use solutions of appropriate salts of the constituting elements of YBCO-based superconductors, that is Yttrium, Barium and Copper, with an atomic ratio of about 1:2:3 in coating techniques for preparing films and coatings of the desired superconductor.
According to one approach a mixture of metal trifluoroacetates was prepared from the respective acetates. However, in the resulting metal trifluoroacetates some acetate and water remained bound which proved to be detrimental for the coating process and the properties of the resulting superconductor film.
For overcoming this problem EP 1 187 231 suggests using a purified starting material for preparing the coating solution having a reduced amount of water and acetate bound. That is, according to EP 1 187 231 a mixture of metal triflouroacetate is prepared with Y, Ba and Cu in a suitable atomic ratio of 1:2:3 which is used for preparing the coating solution. In a first step a metal acetate solution is converted to the corresponding metal trifluoroacetate by applying trifluoroacetic acid, and removing solvent, water and acetate by distillation. However, since some of the water and acetate is not free but bound to the metal trifluoroacetate about 2 to 8% of water and acetate remains in the resulting metal material which is described to be a semitransparent, blue gel or sol of the obtained mixture of metal trifluoroacetate.
In a second purification step the obtained sol like or gel like material of the first step is solved in a lower alcohol such as methanol or ethanol in order to displace water/acetate thereby obtaining a mixture of metal trifluoroacetate with reduced water/acetate content after distillation which is a semitransparent blue gel-like or sol like substance. This purified substance is used for preparing the coating solution for carrying out coating.
According to EP 1 187 231 the starting material for preparing the coating solution must be subjected to a purification process before preparing the coating solution in order to reduce the water/acetate content.
The coating process as disclosed in this prior art is therefore rather lengthy due to the time consuming purification steps necessary before coating.
In US 2004/0071882 precursor solutions for preparing YBa2Cu3O-based superconductor films by coating techniques are disclosed. It is suggested dissolving organic metal salts such as acetates and trifluoroacetates, within a suitable solvent for preparing the coating solutions, wherein the ratio of the molar amount of each of the metals Y:Ba:Cu in the coating solution is 1:2.3.
In the process according to US 2004/0071882 the salts used are freshly metered for each separate run and are immediately dissolved in the solvent. There is no indication of a storable premixture which is already ready for immediate use without the need of metering prior to each run.
Separate metering of the components for each run can lead to differing quality in the final superconductor due to weight failure from run to run. Moreover, in particular in small scale processes with only small amounts of salts even small weight failures can lead to drastical deviations from the desired ideal 1:2:3 ratio for the metals.
It would be desirable to have a ready premixed precursor material comprising the salts of the constituent elements in the desired atomic ratio which can be used immediately on demand without pre-treatment such as purification or metering etcetera, which can be stored even over a long term, is ready to use in the preparation of a coating solution without time-consuming pre-treatment steps, and which reduces the risk of quality variation due to weight failures.
Commercially available is a methanolic solution of metal trifluoroacetate of Yttrium, Barium and Copper. However, sedimentation of basic Copper trifluoroacetate is observed within one week rendering the solution unusable.
Thus, there is a need for a premixed precursor material ready for preparing a coating solution for the preparation of YBCO-based superconductors which has long term storability and can be immediately used for preparing the coating solution without any further pre-treatment.